Impact of elevated atmospheric CO2 on soil bacteria community in a grazed pasture after 12-year enrichment

This study was designed to compare soil bacterial communities under ambient (aCO2) and elevated (eCO2) carbon dioxide after 12 years of enrichment using Free Air Carbon Dioxide Enrichment (FACE) in a grazed grassland. Grazing animals can have profound effects on nutrient cycling through the return of nutrient in excreta and by their influence on plant community composition through diet selection. The abundance and composition of bacterial communities were evaluated by real-time quantitative Polymerase Chain Reaction (qPCR) and pyrosequencing based on the analysis of bacterial 16S rRNA genes. The results showed the overall bacterial community structure was not altered by the eCO2 treatment despite the substantial changes in soil functions, pools and fluxes under eCO2 documented at this site in previous studies. The dominant phyla in both treatments were Actinobacteria, Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes and Planctomycetes, accounting for 87% of the total microbial 16S rRNA sequence reads. At the phylum level, Planctomycetes and Bacteria incertae sedis increased and BRC, Cyanobateria and TM7 decreased significantly at eCO2. Most changes were observed at lower taxonomic levels where the abundance of 30 of the 200 most abundant OTUs were responsive to eCO2 however these changes were not sufficient to differentiate the overall communities. It remains uncertain whether these changes in the lower order taxa could be responsible for the observed changes in soil properties. These first data for a grazed ecosystem are broadly consistent with those froma range of other ecosystems where CO2 effects are confined to relatively few taxa.